This invention relates to radio communication systems and more particularly to measurement of received signal power in such systems.
In the continuing evolution of mobile cellular radio standards like GSM and wideband code division multiple access (WCDMA), new transmission techniques like orthogonal frequency division multiplex (OFDM) will be used in new cellular communication systems. Furthermore, to migrate smoothly from existing cellular systems to new high-capacity, high-data-rate systems in the existing radio spectrum, the new systems have to be able to operate with flexible communication channel bandwidths.
One such new flexible cellular communication system is called Third Generation Long Term Evolution (3G LTE), which is currently being standardized by the Third Generation Partnership Project (3GPP). The 3G LTE specifications can be seen as an evolution of the current WCDMA specifications also promulgated by the 3GPP. A 3G LTE system will use OFDM as a multiple access technique (called OFDMA) in the downlink (DL) from system nodes to user equipments (UEs), will operate with channel bandwidths ranging from 1.25 megahertz (MHz) to 20 MHz, and will support data rates up to 100 megabits per second (Mb/s) on the largest-bandwidth channels. Besides high-data-rate services, 3G LTE systems are expected to provide low-data-rate services, such as speech. Because 3G LTE is designed for packet data according to the familiar transmission control protocol/internet protocol (TCP/IP), it is expected that the service that carries speech will use voice-over-IP (VoIP).
In an OFDMA communication system, the data stream to be transmitted is portioned among a number of narrowband subcarriers that are transmitted in parallel. In general, a resource block devoted to a particular UE is a particular number of particular subcarriers used for a particular period of time. Different groups of subcarriers can be used at different times for different users. Because each subcarrier is narrowband, each carrier experiences mainly flat fading, which makes it easier for a UE to demodulate each subcarrier. OFDMA communication systems are described in the literature, for example, U.S. Patent Application Publication No. US 2008/0031368 A1 by B. Lindoff et al.
FIG. 1 depicts a typical cellular communication system 10. Radio network controllers (RNCs) 12, 14 control various radio network functions, including for example radio access bearer setup, diversity handover, etc. In general, each RNC directs calls to and from a UE, such as a mobile station (MS), mobile phone, or other remote terminal, via appropriate base station(s) (BSs), which communicate with each other through DL (or forward) and uplink (UL, or reverse) channels. In FIG. 1, RNC 12 is shown coupled to BSs 16, 18, 20, and RNC 14 is shown coupled to BSs 22, 24, 26.
Each BS, or Node B in 3G vocabulary, serves a geographical area that is divided into one or more cell(s). In FIG. 1, BS 26 is shown as having five antenna sectors S1-S5, which can be said to make up the cell of the BS 26, although a sector or other area served by signals from a BS can also be called a cell. In addition, a BS may use more than one antenna to transmit signals to a UE. The BSs are typically coupled to their corresponding RNCs by dedicated telephone lines, optical fiber links, microwave links, etc. The RNCs 12, 14 are connected with external networks such as the public switched telephone network (PSTN), the internet, etc. through one or more core network nodes, such as a mobile switching center (not shown) and/or a packet radio service node (not shown).
It should be understood that the arrangement of functionalities depicted in FIG. 1 can be modified in 3G LTE and other communication systems. For example, the functionality of the RNCs 12, 14 can be moved to the Node Bs 22, 24, 26, and other functionalities can be moved to other nodes in the network. It will also be understood that a base station can use multiple transmit antennas to transmit information into a cell/sector/area, and those different transmit antennas can send respective, different pilot signals.
FIG. 2 is a frequency-vs.-time plot showing an arrangement of DL subcarriers in an OFDM communication system, such as a 3G LTE system. As shown in FIG. 2, a resource block includes twelve subcarriers spaced apart by fifteen kilohertz (kHz), which together occupy approximately 180 kHz in frequency and 0.5 millisecond (ms) in time, or one time slot. FIG. 2 shows each time slot including seven OFDM symbols, or resource elements (REs), each of which has a short (normal) cyclic prefix, although six OFDM symbols having long (extended) cyclic prefixes can also be used in a time slot. It will be understood that resource blocks can include various numbers of subcarriers for various periods of time.
An important aspect of a 3G LTE system is the mobility of the UEs, and so fast and efficient cell search and received signal power measurements are important for a UE to get and stay connected to a suitable cell, which can be called the “serving cell”, and to be handed over from one serving cell to another. In current 3G LTE specifications, handover decisions are based on measurements of reference signals and symbols received power (RSRP), which can be defined as the average received signal power of reference symbols or signals (RS) transmitted by a Node B. A UE measures RSRP on its serving cell as well as on neighboring cells that the UE has detected as a result of a specified cell search procedure.
The RS, or pilots, are transmitted from each Node B at known frequencies and time instants, and are used by UEs for synchronization and other purposes besides handover. Such reference signals and symbols are described for example in Section 7.1.1.2.2 of 3GPP Technical Report (TR) 25.814 V7.0.0, Physical Layer Aspects for Evolved Universal Terrestrial Radio Access (UTRA) (Release 7), June 2006, and Sections 6.10 and 6.11 of 3GPP Technical Specification (TS) 36.211 V8.1.0, Physical Channels and Modulation (Release 8), November 2007.
RS are transmitted from each of possibly 1, 2, or 4 transmit antennas of a Node B on particular REs that can be conveniently represented on the frequency-vs.-time plane as depicted in FIG. 3. It will be understood that the arrangement of FIG. 3 is just an example and that other arrangements can be used.
FIG. 3 shows two successive time slots, indicated by the vertical solid lines, which can be called a sub-frame. The frequency range depicted in FIG. 3 includes about twenty-six subcarriers, only nine of which are explicitly indicated. RS transmitted by a first transmit (TX) antenna of a Node B are denoted R and by a possible second TX antenna in the node are denoted by S. In FIG. 3, RS are depicted as transmitted on every sixth subcarrier in OFDM symbol 0 and OFDM symbol 3 or 4 (depending on whether the symbols have long or short cyclic prefixes) in every slot. Also in FIG. 3, the RSs in symbols 3 or 4 are offset by 3 subcarriers relative to the RS in OFDM symbol 0, the first OFDM symbol in a slot.
The artisan will understand that it is desirable for a UE to base its RSRP measurements on RS transmitted from all TX antennas used by a node for a cell. Nevertheless, the number of TX antennas used by a cell detected in a cell search procedure but not currently connected to the UE (which can be called “a detected neighboring cell”) is typically not known in advance by the UE. As a result, the UE needs to determine the number of TX antennas “blindly”, i.e., without receiving a special message informing the UE of that number. When signals transmitted by a cell are received by a UE with low signal-to-interference ratios (SIRs), the risk of blindly detecting an erroneous number of TX antennas is significant. Low SIR is a common situation for a detected neighboring cell because such a cell's signal power level at the UE is usually lower than the received power level of the serving cell. Due to the increased probability of a blind detection error, RSRP measurements based on an assumed second TX antenna in low SIR conditions are unreliable with a potentially large bias.
One solution to this problem would be to have the UE execute a more computationally intensive blind detection algorithm in an effort to reduce the probability of its determining the wrong number of Node B TX antennas. Such an algorithm would typically require fast Fourier transform (FFT) processing of neighboring cell data. FFT processing consumes time, power, and/or hardware resources that are limited in many UEs and increases the complexity of the signal-power-estimate processing in a UE, both of which render this solution undesirable.
Furthermore, a UE typically assumes that the characteristics of the DL channel are constant over a number of subcarriers (i.e., the channel is constant with frequency) and over a number of OFDM symbols (i.e., the channel is constant in time). Based on that assumption, the UE estimates the RSRP by coherently averaging received symbols over such a “constant” group to get a channel estimate Hi for a subcarrier i, computes the square of the absolute value of the channel estimate |Hi|2 to obtain a received signal power estimate over the “constant” group of symbols, and then computes a non-coherent average of such signal power estimates over several groups, e.g., an entire channel bandwidth, to determine an RSRP measurement (estimate). Two such assumed “constant” groups are indicated in FIG. 3 by the dashed lines.
In the arrangement depicted in FIG. 3, such coherent averaging to estimate the RSRP can proceed as follows. The UE's baseband signal Yi corresponding to RS from TX antenna 1 can be written as follows:Yi=Hi1Ri+Ei  Eq. 1and the UE's baseband signal corresponding to RS from a possible TX antenna 2 can be written similarly as follows:Yi=Hi2Si+Ei  Eq. 2Coherent averaging of a number M of received reference symbols followed by non-coherent averaging of N coherent averages can be written as follows:
                              S                      e            ⁢                                                  ⁢            s            ⁢                                                  ⁢            t                          =                              1            N                    ⁢                                    ∑                              n                =                1                            N                        ⁢                                                                                                1                    M                                    ⁢                                                            ∑                                              m                        =                        1                                            M                                        ⁢                                          R                      ⁢                                                                                          ⁢                                              S                        m                                                  e                          ⁢                                                                                                          ⁢                          s                          ⁢                                                                                                          ⁢                          t                                                                                                                                                n              2                                                          Eq        .                                  ⁢        3            in which Sest is the RSRP measurement (estimate) and RSest are the estimated RS symbols.
The unreliability and potentially large bias of RSRP measurements based on an assumed second TX antenna in low SIR conditions can now be understood for the exemplary case of additive white Gaussian noise, i.e., a static channel impaired by only addition of wideband noise having a constant spectral density. The received signal power estimate Sest described by Eq. 3 can approximated by:Sest=S+1/M  Eq. 4in which S is the received signal power. If S is large, then Sest=S in Eq. 4, and if S is small, then Sest=1/M, which is to say Sest has a bias. The number N affects only the variance, not the bias of the estimate, as explained in U.S. Patent Application Publication No. US 2008/0101488 A1 by L. Wilhelmsson et al. for “Robust and Low-Complexity Combined Signal Power Estimation”, for example.
In addition, the DL channel commonly suffers from delay spread and Doppler shift, and so the channel is not constant as typically assumed, leading again to increased probability of biassed RSRP measurement values. A known solution to this problem of varying DL channels is to use more advanced methods of estimating the channel and signal power (e.g., methods based on Wiener filtering). Like the FFT processing mentioned above, such more advanced methods are computationally intensive and need to be done on each detected neighboring cell, rendering this solution undesirable.
Therefore, there is a need for improved methods and apparatus for measuring signal power of cells without significantly increasing the complexity of the measurements.